1. Field of the Invention
One of the aspects of present invention relates to a technique for categorizing input spectral data into a group having close characteristics.
2. Description of the Related Art
In recent years, devices handling images such as digital cameras, displays, and printers have undergone evolution, and a multi-primary color technique has been under development as a technology development relating to color reproduction. Particularly in image input devices such as digital cameras, technology development relating to a multi-band camera that takes a picture using four color filters or more exceeding the number of conventional three primary colors (red, green, and blue) color filters is in progress.
One advantage of the technology to take a picture of a subject by a multi-band camera is that spectral data of the subject, that is, the spectral reflectance can be estimated with high precision. The spectral reflectance is subject-specific color information that does not depend on illumination information, and coloring of the subject under an illumination light source that is different from the light source when an image is captured can be estimated with high precision by acquiring spectral data.
In addition to color reproduction, a detailed analysis of color information can be performed using the spectral data. FIG. 10 illustrates an example of two spectral reflectances. The spectral reflectances illustrated in FIG. 10 match in tristimulus values under a specific light source (D50). In other words, colors cannot be distinguished when XYZ values are compared. However, as is evident from FIG. 10, two pieces of spectral data are significantly different and so differences of colors that cannot be determined by using the XYZ values can be determined by analyzing the spectral data.
According to Japanese Patent Application Laid-Open No. 2006-53070, the types of pigments used in coloring of the cultural properties are analyzed by the spectral data in color analysis of cultural properties. Identification of coloring materials used in coloring of the cultural properties can be effective information for, for example, recovery of fading or restoration work.
When compared with the RGB values, XYZ values, or Lab values, spectral data has a huge amount of data (dimensionality) needed to represent one color. While the RGB values, XYZ values, and Lab values are three-dimensional data representing one color by three values of X, Y, and Z, spectral data is 31-dimensional data when the wavelength range of 400 nm to 700 nm is sampled at intervals of 10 nm.
According to the technology discussed in Japanese Patent Application Laid-Open No. 2006-53070, pigments used for coloring of a colored member are estimated by comparing the spectral reflectance of the colored member with spectral data stored in advance. However, spectral data has a huge dimensionality when compared with tristimulus value data such as XYZ data and thus, the spectral reflectance of the colored member is considered to rarely match the spectral data stored in advance. Therefore, an erroneous determination may be made if the spectral reflectances are simply compared.